The present invention relates generally to waveguides and, more particularly, relates to waveguides including at least one photonic crystal region for directing signals propagating therethrough such as to form a ninety-degree bend, beamsplitter, polarizing beamsplitter, Mach-Zender interferometer, ring resonator or the like.
Conventional waveguides, such as Silica waveguides, composed of a core and a cladding with low refractive index difference are an attractive technology platform for planar lightwave circuits (PLC""s) because of their straightforward design for single mode operation, low propagation loss, and low dispersion. One of the research interests in PLC""s is to increase their integration level on a single chip. In this regard, compact PLC components are typically required to increase the integration level. But because a relatively large radius of curvature (on the order of mm) is typically necessary to achieve a high efficiency waveguide bend, the sizes of such waveguide-based PLC""s are conventionally quite large (cm range), which can limit the possible integration level of such PLC""s.
Photonic crystals (PhC""s), typically comprising arrays of periodic dielectric material embedded in a homogeneous material, have been the focus of the intense research because PhC""s can be used to achieve ultracompact PLC""s. In this regard, defects in PhC""s, where the periodicity is broken, can support modes for frequencies in the photonic band gap of the PhC""s (frequency ranges prohibited from propagation in PhC). Since PhC defect modes are well confined in the defect region, ultracompact waveguide and sharp bend structures can be obtained from PhC""s. One such waveguide structure is disclosed by U.S. Pat. No. 6,134,369 to Kurosawa, entitled: Compact Optical Waveguide. As disclosed by the Kurosawa patent, an optical waveguide includes a photonic band gap element as a reflector to enable light to be reflected at angles greater than a critical angle. The waveguide of the Kurosawa patent includes a waveguide channel that has a two-dimensional photonic band gap element on each of the inside and outside of a bend in the waveguide channel. In this regard, the two-dimensional photonic band gap elements form reflective surfaces at the corners of the waveguide channel.
Whereas waveguides such as that disclosed by the Kurosawa patent are adequate in realizing a sharp bend, such waveguides have drawbacks. In this regard, it is typically desirable for the PhC""s of such conventional PhC waveguides to have a wide band gap to confine signals within the waveguide. Also, as is well known to those skilled in the art, the larger the difference between the refractive indices of the PhC""s and the surrounding material, the wider the band gap of the PhC. Conventional PhC waveguides, then, also typically comprise PhC""s and surrounding material that have a large difference between their respective refractive indices. Thus, such conventional PhC waveguides typically place undesirable design constraints on the PhC region. In this regard, conventional PhC waveguides typically require PhC regions with a full band gap and, in turn, PhC regions and surrounding material having a large refractive index difference, to thereby efficiently operate.
As will also be appreciated, the incident light mode width of signals propagating in the waveguide region can be wider than the width of the conventional PhC waveguide channel. Conventional PhC waveguides, such as that disclosed by the Kurosawa patent, however, typically only include a PhC region that extends to the boundary of the waveguide channel and surrounding cladding material which, in turn, require coupling of signals from the waveguide into the conventional PhC waveguide. Since signals in the waveguide have to be coupled into the smaller width PhC waveguide to manipulate its propagation direction, conventional PhC waveguides can suffer from loss of the mode tail in the cladding by the scattering of signals with the PhC region.
In light of the foregoing background, embodiments of the present invention provide an improved waveguide including at least one photonic crystal region for directing signals propagating therethrough. More particularly, embodiments of the present invention provide a waveguide assembly that includes a waveguide region and photonic crystal (PhC) region(s) for directing signals propagating through the waveguide region. In this regard, the PhC region(s) augment the waveguide region to reduce overall device size while preserving the traditional advantages of conventional waveguides, such as straightforward design for single mode operation, low propagation loss, and low dispersion. As described more fully below, embodiments of the present invention can therefore provide waveguide assemblies that can be configured for uses such as high efficiency waveguide bend, beamsplitter, polarizing beamsplitter, a Mach-Zender interferometer or a ring resonator.
According to one aspect of the present invention, a waveguide assembly is provided that includes a waveguide region and at least one photonic crystal (PhC) region. The waveguide region includes a longitudinally extending core that has an input channel and at least one output channel, and a cladding at least partially surrounding the core for confining signals within the core. The PhC region(s), in turn, extend laterally through at least a portion of the core to at least partially direct signals propagating through the core. More particularly, for example, the PhC region(s) can extend through at least a portion of the core to thereby form a bend, beamsplitter, polarizing beamsplitter, Mach-Zender interferometer or ring resonator for signals propagating through the core.
In contrast to the waveguide assembly of this embodiment of the present invention, conventional PhC waveguides, such as that disclosed by the Kurosawa patent, utilize PhC regions that form a portion of the waveguide, typically the bend in the waveguide. By extending the PhC region(s) at least partially through the core, at least a portion of signals propagating through the input channel can become incident at an angle of incidence on a surface of the PhC region(s) in a manner such that the signals can have a wavelength outside a band gap of the PhC region(s). In this regard, the waveguide assembly of embodiments of the present invention is capable of efficiently operating with signals having wavelengths outside the band gap of the PhC region(s). As also described in more detail below, the PhC region(s) can also extend laterally through at least a portion of the cladding. As such, and further in contrast to conventional PhC waveguides, the PhC region(s) can be capable of covering the incident light mode width of a signal propagating through the waveguide region.
Each PhC region can include a boundary layer at a boundary of the PhC region and the core of the waveguide region. In this regard, the boundary layer is capable of being modified to thereby manipulate a diffraction effect caused by a periodicity at the boundary of the at least one PhC region and the core, particularly when the waveguide is configured as a bend. For example, the boundary layer can be modified by changing the radius, periodicity and/or position of holes or posts that make up the boundary layer, and more generally the PhC region(s).
When the waveguide assembly is configured as a polarizing beamsplitter, a portion of the core through which the PhC region(s) extend can be configured to follow a propagation direction of polarized signals propagating through the core. More particularly, a portion of a first output channel through which the PhC region extends can be sloped with respect to an input channel.
Embodiments of the present invention therefore provide an improved waveguide assembly that includes PhC region(s) capable of augmenting a waveguide region to reduce overall device size while preserving the traditional advantages of conventional waveguides. The waveguide assembly can advantageously be configured to operate in any of a number of different manners including, for example, a high efficiency waveguide bend, beamsplitter, polarizing beamsplitter, Mach-Zender interferometer or ring resonator. In contrast to conventional PhC waveguides, the PhC region(s) can extend through a portion of the core of the waveguide region. Also, the waveguide assembly of embodiments of the present invention can efficiently operate with signals outside the band gap of the PhC region(s). In addition, and further in contrast to conventional PhC waveguides, the PhC region(s) can be capable of covering the incident light mode width of a signal propagating through the waveguide region without the increased losses suffered by conventional PhC waveguide from the coupling. As such, the waveguide assembly of embodiments of the present invention solve the problems identified by prior waveguides and provide additional advantages.